The goal of this project is to understand how a step in a complex behavior is genetically specified. This fundamental problem will be addressed by studying C. elegans male mating behavior. During mating, the male executes a series of stereotyped sub-behaviors that result in the insertion of his copulatory spicules into his mate and the subsequent transfer of sperm. Although C. elegans male mating behavior consists of many steps, this project will focus on dissecting how the male inserts his spicules into his mate's vulva. Spicule insertion behavior is a simple reflex;however, the male nervous system and musculature must regulate many factors. The neurons and muscles must compute when to initiate the behavior and monitor if the behavioral outcome was successful. If an insertion attempt fails, the circuitry must re-initiate the behavior. If penetration is successful, the circuitry keeps the spicules inserted while the next behavioral step proceeds. Males display spicule insertion behavior only during mating. However, mutations can be generated that will cause males to display this behavior abnormally in the absence of mating cues. One of these mutations disrupts the C. elegans unc-103 gene. Unc-103 is the C. elegans homolog of the human h-erg-encoded voltage-gated delayed rectifying K+ channel. In humans, HERG channels regulate cardiac rhythm. Mutations in h-erg that reduce ion channel function can increase the probability of spontaneous lethal heart arrhythmias, a condition called long QT syndrome. Spicule insertion behavior requires coordination between different neurons and muscles. Mutations in unc-103 disrupt that regulation and result in spontaneous seizures of male-specific muscles that are used for copulation. To understand how unc-103 is used to regulate behavior, this project will identify where unc-103 is acting in the behavioral circuitry of males. The project will also determine the identities of mutations that affect candidate genes that may act with or in parallel to unc-103 to regulate behavior. Additionally, this project will identify components of activating pathways that must be attenuated by unc-103 during periods between mating.